What is the Closest Planet to Earth? - Universe Today Universe Today What is the Closest Planet to Earth? Article Updated: 23 Dec , 2016 by Matt Williams A common question when looking at the Solar System and Earth’s place in the grand scheme of it is “which planet is closest to Earth?” Aside from satisfying a person’s general curiosity, this question is also of great importance when it comes to space exploration. And as humanity contemplates mounting manned missions to neighboring planets, it also becomes one of immense practicality. If, someday, we hope to explore, settle, and colonize other worlds, which would make for the shortest trip? Invariable, the answer is Venus . Often referred to as “ Earth’s Twin “, Venus has many similarities to Earth. It is a terrestrial planet, it orbits within the Sun’s habitable zone, and it has an atmosphere that is believed to have once been like Earth’s. Combined with its proximity to us, its little wonder we consider it our twin.

Venus’ Orbit: Venus orbits the Sun at an average distance (semi-major axis) of 108,208,000 km (0.723 AUs), ranging between 107,477,000 km (0.718 AU) at perihelion and 108,939,000 km (0.728 AU) at aphelion. This makes Venus’ orbit the least eccentric of all the planets in the Solar System. In fact, with an eccentricity of less than 0.01, its orbit is almost circular. Earth and Venus’ orbit compared. Credit: Sky and Telescope When Venus lies between Earth and the Sun, it experiences what is known as an inferior conjunction. It is at this point that it makes its closest approach to Earth (and that of any planet) with an average distance of 41 million km (25,476,219 mi). On average, Venus achieves an inferior conjunction with Earth every 584 days. And because of the decreasing eccentricity of Earth’s orbit, the minimum distances will become greater over the next tens of thousands of years.

So not only is it Earth’s closest neighbor (when it makes its closest approach), but it will continue to get cozier with us as time goes on! Venus vs. Mars: As Earth’s other neighbor, Mars also has a “close” relationship with Earth. Orbiting our Sun at an average distance of 227,939,200 km (1.52 AU), Mars’ highly eccentric orbit (0.0934) takes it from a distance of 206,700,000 km (1.38 AU) at perihelion to 249,200,000 km (1.666 AU) at aphelion. This makes its orbit one of the more eccentric in our Solar System, second only to Mercury For Earth and Mars to be at their closest, both planets needs to be on the same side of the Sun, Mars needs to be at its closest distance from the Sun (perihelion), and Earth needs to be at its farthest (aphelion).

This is known as opposition, a time when Mars appears as one of the brightest objects in the sky (as a red star), rivaling that of Venus or Jupiter. The eccentricity in Mars’ orbit means that it is . Credit: NASA But even at this point, the distance between Mars and Earth ranges considerably. The closest approach to take place occurred back in 2003, when Earth and Mars were only 56 million km (3,4796,787 mi) apart. And this was the closest they’d been in 50,000 years. The next closest approach will take place on July 27th, 20178, when Earth and Mars will be at a distance of 57.6 million km (35.8 mi) from each other. It has also been estimated that the closest theoretical approach would take place at a distance of 54.6 million km (33.9 million mi). However, no such approach has been documented in all of recorded history. One would be forced to wonder then why so much of humanity’s exploration efforts (past, present and future) are aimed at Mars.

But when one considers just how horrible Venus’ environment is in comparison, the answer becomes clear. Exploration Efforts: The study and exploration of Venus has been difficult over the years, owing to the combination of its dense atmosphere and harsh surface environment. Its surface has been imaged only in recent history, thanks to the development of radar imaging. However, many robotic spacecraft and even a few landers have made the journey and discovered much about Earth’s closest neighbor. The first attempts were made by the Soviets in the 1960s through the Venera Program . Whereas the first mission ( Venera-1 ) failed due to loss of contact, the second ( Venera-3 ) became the first man-made object to enter the atmosphere and strike the surface of another planet (on March 1st, 1966). This was followed by the Venera-4 spacecraft, which launched on June 12th, 1967, and reached the planet roughly four months later (on October 18th). The first color pictures taken of the surface of Venus by the Venera-13 space probe. Credit: NASA NASA conducted similar missions under the Mariner program.

The Mariner 2 mission, which launched on December 14th, 1962, became the first successful interplanetary mission and passed within 34,833 km (21,644 mi) of Venus’ surface. Between the late 60s and mid 70s, NASA conducted several more flybys using Mariner probes – such as the Mariner 5 mission on Oct. 19th, 1967 and the Mariner 10 mission on Feb. 5th, 1974. The Soviets launched six more Venera probes between the late 60s and 1975, and four additional missions between the late 70s and early 80s. Venera-5 , Venera-6 , and Venera-7 all entered Venus’ atmosphere and returned critical data to Earth. Venera 11 and Venera 12 detected Venusian electrical storms; and Venera 13 and Venera 14 landed on the planet and took the first color photographs of the surface.

The program came to a close in October 1983, when Venera 15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with synthetic aperture radar. By the late seventies, NASA commenced the Pioneer Venus Project , which consisted of two separate missions. The first was the Pioneer Venus Orbiter , which inserted into an elliptical orbit around Venus (Dec. 4th, 1978) to study its atmosphere and map the surface. The second, the Pioneer Venus Multiprobe , released four probes which entered the atmosphere on Dec. 9th, 1978, returning data on its composition, winds and heat fluxes. Artist’s impression of NASA’s Pioneer Venus Orbiter in orbit around Venus. Credit: NASA In 1985, the Soviets participated in a collaborative venture with several European states to launch the Vega Program . This two-spacecraft initiative was intended to take advantage of the appearance of Halley’s Comet in the inner Solar System, and combine a mission to it with a flyby of Venus.

While en route to Halley on June 11th and 15th, the two Vega spacecraft dropped Venera-style probes into Venus’ atmosphere to map its weather. NASA’s Magellan spacecraft was launched on May 4th, 1989, with a mission to map the surface of Venus with radar. In the course of its four and a half year mission, Magellan provided the most high-resolution images to date of the planet, was able to map 98% of the surface and 95% of its gravity field. In 1994, at the end of its mission, Magellan was sent to its destruction into the atmosphere of Venus to quantify its density. Venus was observed by the Galileo and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan was the last dedicated mission to Venus for over a decade. It was not until October of 2006 and June of 2007 that the MESSENGER probe would conduct a flyby of Venus (and collect data) in order to slow its trajectory for an eventual orbital insertion of Mercury.

The Venus Express , a probe designed and built by the European Space Agency, successfully assumed polar orbit around Venus on April 11th, 2006. This probe conducted a detailed study of the Venusian atmosphere and clouds, and discovered an ozone layer and a swirling double-vortex at the south pole before concluding its mission in December of 2014 . Since December 7th, 2015, Japan’s Akatsuki has been in a highly elliptical Venusian orbit. Because of its hostile surface and atmospheric conditions, Venus has proven to be a tough nut to crack, despite its proximity to Earth. In spite of that, NASA, Roscosmos, and India’s ISRO all have plans for sending additional missions to Venus in the coming years to learn more about our twin planet. And as the century progresses, and if certain people get their way, we may even attempt to send human colonists there ! If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth . And here’s a link to NASA’s Earth Observatory . Astronomy Cast also has an interesting episode on the subject. Listen here, Episode 50: Venus .

Share this: If Venus once had an atmosphere similar to Earth, what happened? I understand the average temperature on Venus now exceeds a uniform 900 degrees Fahrenheit over the entire planet! Helio George Planet Earth: Facts About Its Orbit, Atmosphere & Size Planet Earth: Facts About Its Orbit, Atmosphere & Size By Charles Q. Choi, Space.com Contributor | December 11, 2014 10:49pm ET MORE Credit: NASA Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life. Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants, Jupiter , Saturn , Uranus and Neptune , but larger than the three other rocky planets, Mercury , Mars and Venus . Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator.

Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of Earth's atmosphere is made up of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space . Orbital characteristics Earth spins on an imaginary line called an axis that runs from the North Pole to the South Pole, while also orbiting the sun. It takes Earth 23.439 hours to complete a rotation on its axis, and roughly 365.26 days to complete an orbit around the sun. Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons . Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets.

Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.

Orbit & rotation Some statistics about Earth, according to NASA : Average distance from the sun: 92,956,050 miles (149,598,262 km) Perihelion (closest approach to the sun): 91,402,640 miles (147,098,291 km) Aphelion (farthest distance from the sun): 94,509,460 miles (152,098,233 km) Length of solar day (single rotation on its axis): 23.934 hours Length of year (single revolution around the sun): 365.26 days Equatorial inclination to orbit: 23.4393 degrees Earth's formation and evolution Scientists think Earth was formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula . As the nebula collapsed because of its gravity, it spun faster and flattened into a disk.

Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form ever-larger bodies, including Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets. Scientists think Earth started off as a waterless mass of rock . Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape. The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian . Evidence for life has been found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.

The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals. Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well. The Mesozoic saw the ascendence of dinosaurs , although mammals also appear in the fossil record about 200 million years ago. During this time, flowering plants became the dominant plant group and continue to be so today. The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact.

Mammals survived to become the dominant land animals of today. Composition & structure Atmosphere Earth's atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us. Air surrounds Earth and becomes thinner farther from the surface. Roughly 100 miles (160 km) above Earth, the air is so thin that satellites can zip through with little resistance. Still, traces of atmosphere can be found as high as 370 miles (600 km) above the surface. The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion, causing the weather. Sunlight heats the planet's surface, causing warm air to rise. This air ultimately expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks, only to get warmed by the Earth once again. Above the troposphere, some 30 miles (48 km) above the Earth's surface, is the stratosphere.

The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface. Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth. Without this so-called " greenhouse effect ," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus. Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling. Magnetic field The northern lights are more formally known as auroras, and are caused by interactions between the solar wind and the Earth's magnetic field. See how the northern lights work in this Space.com infographic . Credit: Karl Tate, SPACE.com Contributor Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic North Pole recently accelerating its northward motion to 24 miles (40 km) annually, likely exiting North America and reaching Siberia in a few decades.

Earth's magnetic field is changing in other ways, too — globally, the magnetic field has weakened 10 percent since the 19th century , according to NASA. These changes are mild compared to what Earth's magnetic field has done in the past — sometimes the field completely flips, with the north and the south poles swapping places. When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow, a phenomenon known as the aurorae , the northern and southern lights. Chemical composition Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium , potassium , and magnesium at about 2 percent each. Earth's core consists mostly of iron and nickel and potentially smaller amounts of lighter elements such as sulfur and oxygen. The mantle is made of iron and magnesium-rich silicate rocks.

(The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals.) Internal structure Earth's core is about 4,400 miles (7,100 km) wide, slightly larger than half the Earth's diameter and roughly the size of Mars . The outermost 1,400 miles (2,250 km) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 km) in diameter — is solid. Above the core is Earth's mantle, which is about 1,800 miles (2,900 km) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges. Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt.

Continental crust averages some 25 miles (40 km) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 km) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf. Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degree C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.

Recent exoplanet surveys such as NASA’s Kepler mission suggest that Earth-size planets are common throughout the Milky Way galaxy. Nearly a fourth of sun-like stars observed by Kepler have potentially habitable Earth-size planets . Earth's moon Earth's moon is 2,159 miles (3,474 km) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more. The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars. Species overview Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million and 100 million species on Earth , but science has only identified about 2 million of them.

Earth is the only body in the solar system known to host life, although scientists suspect that other candidates — such as Saturn’s moon Titan or Jupiter’s moon Europa — have the potential to house primitive living creatures. Scientists have yet to precisely nail down exactly how complex life rapidly evolved on Earth from more primitive ancestors. One solution suggests that life first evolved on the nearby planet Mars, once a habitable planet, then traveled to Earth on meteorites hurled from the Red Planet. Additional reporting by Nola Taylor Redd, Space.com Contributor Related: Rex Parker Does the NYT Crossword Puzzle: Point in planet's orbit that's closest to sun / WED 9-22-10 / Sanctuary fixture / Kansas canine / Turn of millennium explorer Point in planet's orbit that's closest to sun / WED 9-22-10 / Sanctuary fixture / Kansas canine / Turn of millennium explorer Constructor: Peter A.

Collins Relative difficulty: Medium-Challenging THEME: EIEIO — four theme answers contain those five vowels (in order); theme revealed by OLD MACDONALD (57A: Subject of a children's song associated with the vowels in the answer to each starred clue) Word of the Day: PERIHELION (46A: *Point in a planet's orbit that's closest to the sun) — n. The point nearest the sun in the orbit of a planet or other celestial body. [Alteration of New Latin perihēlium : + Greek hēlios, sun.] (answers.com) • • • Cute idea for a theme, but when you have to go to PERIHELION for your fourth theme answer, you should ... keep searching for a fourth theme answer. Jarring to go from common to common to common to WTF!?!?!? I can't be the only one who's never even seen the word before. Surely there is a list somewhere at some puzzle/word nerd site of words and phrases that have this particular vowel sequence. I can't help but wonder if there isn't another path to the execution of this theme that doesn't involve such an obscure (in everyday language) word.

It's a cool word, don't get me wrong. I'd love to see it in a late-week themeless. It's icky here, though, because it feels forced. "Here's the only other 9-letter word or phrase I could get to do this." Less than ideal. Also less than ideal is the fill. Grid seems Very lazily filled. Just because a section is tiny doesn't mean it doesn't matter. Just changing MOOS (58D: Sounds heard by 57-Across) to MOPE makes the south instantly much better (two letters changed, three words improved). I get that those MOOS and BAAS are (maybe) supposed to be bonus theme answers, but come on, they're just common (and annoying) plurals that we see in puzzles every day. Load the puzzle up with animal sounds, or else lose MOOS and BAAS in order to make the grid better. That LADED (65A: Put on, as cargo) / ILENE (68A: "Mr.

Belvedere" actress Graff) / ARLENE (51D: Francis of "What's My Line?") (all yuck) section Has to be improvable as well. MALAY (1A: Dweller on an Asian peninsula) / A MOLE (?) (14A: Whack-___) / YESES (5D: Some survey responses) / ALEF (4D: Beth preceder), same thing. A little less churning out, a little more thought to the small details—that would be nice.

Theme answers: 20A: *Turn-of-the-millennium explorer (LEIF ERICSSON) 30A: *1996 Grammy winner for the album "Falling Into You" (CELINE DION) 38A: *Treaty of Versailles signer (PRESIDENT WILSON) 46A: *Point in a planet's orbit that's closest to the sun (PERIHELION) Difficulties for me included PERIHELION, obviously; the HEART part of LOSING HEART (3D: Becoming discouraged), which I just couldn't see (possibly because I had NAH for HAH, 34A: "Not a chance!"); HIGH ALTAR, the clue for which meant nothing to me (35D: Sanctuary feature)—is a "sanctuary" a specific architectural feature of a Catholic church? I just don't know. I also had trouble getting the (graphic) RIB SPREADER (26D: Tool used in thoracic surgery). Everything else was easy, but those hiccups were enough to put me into an above-average time.

Bullets: 37A: 1,055 joules: Abbr. (BTU) — reflexively wrote in ERG. Physics 3 letters=ERG. One of the perils of putting brain on autofillpilot: occasional wrongness. 44A: Pitcher Maddux who won four straight Cy Young Awards (GREG) — arguably the best pitcher in baseball for a good stretch of the '90s. In addition to his many pitching accomplishments, he won 18 (not a typo) Gold Glove Awards. Ridiculous. He is sure to be a first-ballot Hall-of-Famer. 11D: Tool used by Hansel and Gretel's father (AXE) — another peril of brain on autofillpilot. Tool, three letters, starts with "A" — now AXE is a good bet, but my brain went AWL! Pretty sure their father was not a cobbler... 1D: Catch that might be mounted (MARLIN) — tricky, odd-sounding clue, but I got it easily. Weird how I can nail this and yet muff LOSING HEART.

42D: It was dropped in the '60s (LSD) — and the '70s. And the '80s. Etc. 38D: Any singer with Gladys Knight (PIP) — I've seen people sing with her who were not PIPs, so not *any* singer. I looove Gladys. Happy birthday to my not-so-little girl. Double digits. Holy cow. I think that's officially tweendom. Signed, Rex Parker, King of CrossWorld Pluto: Facts & Information About the Dwarf Planet Pluto A wide view of Pluto taken by the New Horizons space probe in July 2015. Credit: NASA/APL/SwRI Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar system. It is also one of the largest known members of the Kuiper Belt , a shadowy disklike zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets.

In 2006, Pluto was reclassified as a dwarf planet , a change widely thought of as a demotion. The question of Pluto's planet status has attracted controversy and stirred debate in the scientific community , and among the general public , since then. American astronomer Percival Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at these two planets from beyond. Lowell predicted the mystery planet's location in 1915, but died without finding it. Pluto was finally discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers. Pluto got its name from an 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto.

Physical characteristics Since Pluto is so far from Earth, little was known about the dwarf planet's size or surface conditions until 2015, when NASA's New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide as Earth's moon. Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features , including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice. [ Photos of Pluto and Its Moons ] Pluto's surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet's surface.

Another distinct feature on Pluto's surface is a large heart-shaped region known unofficially as Tombaugh Reggio (after Clyde Tombaugh). The left side of the reggio (an area that takes on the shape of an ice cream cone) is covered in carbon monoxide ice. Other variations in the composition of surface materials have been identified within the "heart" of Pluto. In the center left of Tombaugh Reggio is a very smooth region unofficially known by the New Horizons team as "Sputnik Planum," after Earth's first artificial satellite, Sputnik. This region of Pluto's surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It's possible that this region is still being shaped and changed by geologic processes . These icy plains also display dark streaks that are a few miles long, and aligned in the same direction. It's possible the lines are created by harsh winds blowing across the dwarf planet's surface. NASA's Hubble Space Telescope has also revealed evidence that Pluto's crust could contain complex organic molecules .

Pluto's surface is one of the coldest places in the solar system, at roughly minus 375 degrees Fahrenheit (minus 225 degrees Celsius). When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes. Orbital characteristics Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. Since the dwarf planet's orbit is so eccentric, or far from circular, Pluto's distance from the sun can vary considerably. The dwarf planet actually gets closer to the sun than Neptune is for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world. As a result of that orbit, after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet). When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane.

Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds. A close-up view of the surface of Pluto, taken by the New Horizons space probe in July, 2015, revealed the presence of icy mountains on the dwarf planet's surface. Credit: NASA-JHUAPL-SwRI Composition & structure Atmospheric composition : Methane, nitrogen. Observations by New Horizons show that Pluto's atmosphere extends as far as 1,000 miles (1,600 km) above the surface of the dwarf planet. Magnetic field: It remains unknown whether Pluto has a magnetic field, but the dwarf planet's small size and slow rotation suggest it has little to no such field. Chemical composition: Pluto probably consists of a mixture of 70 percent rock and 30 percent water ice.

Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface. Orbit & rotation Pluto's rotation is retrograde compared to the solar systems' other worlds; it spins backward, from east to west.

Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth Pluto's moons Pluto has five moons: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closest to Pluto and Hydra the most distant. In 1978, astronomers discovered that Pluto had a very large moon nearly half the dwarf planet's own size. This moon was dubbed Charon, after the mythological demon who ferried souls to the underworld in Greek mythology.

Because Charon and Pluto are so similar in size, their orbit is unlike that of most planets and their moons. Both Pluto and Charon orbit a point in space that lies between them, similar to the orbits of binary star systems , For this reason, scientists refer to Pluto and Charon as a double dwarf planet, double planet or binary system. Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking. While Pluto has a reddish tint, Charon appears more grayish. In its early days, the moon may have contained a subsurface ocean , though the satellite probably can't support one today. Compared with most of the solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun.

Observations of Charon by New Horizons have revealed the presence of canyons on the moon's surface. The deepest of those canyons plunge downward for up to 6 miles (9.7 km). A long swatch of cliffs and troughs stretches for 600 miles (970 km) across the middle of the satellite. A section of the moon's surface near one pole is covered in a much darker material than the rest of the planet. Similar to regions of Pluto, much of Charon's surface is free of craters — suggesting the surface is quite young and geologically active . In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission and discovered two other tiny moons of Pluto , now dubbed Nix and Hydra. These satellites are two and three times farther away from Pluto than is Charon. Based on measurements by New Horizons, Nix is estimated to be 26 miles (42 km) long and 22 miles (36 km) wide, while Hydra is estimated at 34 miles (55 km) long and 25 miles (40 km) wide.

It is likely that Hydra's surface is coated primarily in water ice. Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. On July 11, 2012, a fifth moon , Styx, was discovered (with an estimated width of 6 miles or 10 km), further fueling the debate about Pluto's status as a planet. The four newly spotted moons may have formed from the collision that created Charon. Their orbits have been found to be highly chaotic . Research & exploration NASA's New Horizons mission is the first probe to study Pluto, its moons and other worlds within the Kuiper Belt up close. It was launched on January 2006, and successfully made its closest approach to Pluto on July 14, 2015. It will take 16 months to download all the data that the spacecraft collected during its flyby of Pluto and its moons. Mission leaders say they would like to send New Horizons to study another object in the Kuiper belt, but NASA has not yet approved an extended mission.

The New Horizons probe carries some of the ashes of Pluto's discoverer , Clyde Tombaugh. The limited knowledge of the Pluto system created unprecedented dangers for the New Horizons probe. Prior to the mission's launch, scientists knew of the existence of only three moons around Pluto. The discovery of Kerberos and Styx during the spacecraft's journey fueled the idea that more satellites could orbit the dwarf planet, unseen from Earth . Collisions with unseen moons, or even small bits of debris, could have seriously damaged the spacecraft. But the New Horizons design team equipped the space probe with tools to protect it during its journey. Pluto's formation & origins The leading hypothesis for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-size object. Most of the combined matter became Pluto, while the rest spun off to become Charon, this idea suggests. [ Infographic: Pluto: A Dwarf Planet Oddity ] Additional reporting by staff writer Calla Cofield and Nola Taylor Redd, Space.com contributor. Additional resources The International Astronomical Union makes its case for demoting Pluto to a dwarf planet. Track the New Horizons spacecraft as it it continues on its journey into the Kuiper Belt.

Learn more about Pluto at NASA's Dwarf Planets page . Related: Kepler's Three Laws | Astronomy 801: Planets, Stars, Galaxies, and the Universe Planets, Stars, Galaxies, and the Universe Credit: Wikimedia Commons First Law Kepler was a sophisticated mathematician, and so the advance that he made in the study of the motion of the planets was to introduce a mathematical foundation for the heliocentric model of the solar system. Where Ptolemy and Copernicus relied on assumptions, such as that the circle is a “perfect” shape and all orbits must be circular, Kepler showed that mathematically a circular orbit could not match the data for Mars, but that an elliptical orbit did match the data! We now refer to the following statement as Kepler’s First Law: The planets orbit the Sun in ellipses with the Sun at one focus (the other focus is empty). For more information about ellipses, you can read in gory mathematical detail the page hosted at Mathworld , and there is also information on ellipses in Wikipedia .

Here is a demonstration of the classic method for drawing an ellipse: Drawing an ellipse Credit: Wikipedia The two thumbtacks in the image represent the two foci of the ellipse, and the string ensures that the sum of the distances from the two foci (the tacks) to the pencil is a constant. Below is another image of an ellipse with the major axis and minor axis defined: Diagram of an ellipse Credit: Wikipedia We know that in a circle, all lines that pass through the center (diameters) are exactly equal in length. However, in an ellipse, lines that you draw through the center vary in length. The line that passes from one end to the other and includes both foci is called the major axis, and this is the longest distance between two points on the ellipse. The line that is perpendicular to the major axis at its center is called the minor axis, and it is the shortest distance between two points on the ellipse. In the image above, the green dots are the foci (equivalent to the tacks in the photo above). The larger the distance between the foci, the larger the eccentricity of the ellipse.

In the limiting case where the foci are on top of each other (an eccentricity of 0), the figure is actually a circle. So you can think of a circle as an ellipse of eccentricity 0. Studies have shown that astronomy textbooks introduce a misconception by showing the planets' orbits as highly eccentric in an effort to be sure to drive home the point that they are ellipses and not circles. In reality the orbits of most planets in our Solar System are very close to circular, with eccentricities of near 0 (e.g., the eccentricity of Earth's orbit is 0.0167). For an animation showing orbits with varying eccentricities, see the eccentricity diagram at "Windows to the Universe." Note that the orbit with an eccentricity of 0.2, which appears nearly circular, is similar to Mercury's, which has the largest eccentricity of any planet in the Solar System. The elliptical orbits diagram at "Windows to the Universe" includes an image with a direct comparison of the eccentricities of several planets, an asteroid, and a comet.

Note that if you follow the Starry Night instructions on the previous page to observe the orbits of Earth and Mars from above, you can also see the shapes of these orbits and how circular they appear. Kepler’s first law has several implications. These are: The distance between a planet and the Sun changes as the planet moves along its orbit. The Sun is offset from the center of the planet’s orbit. Second Law In their models of the Solar System, the Greeks held to the Aristotelian belief that objects in the sky moved at a constant speed in circles because that is their “natural motion.” However, Kepler’s second law (sometimes referred to as the Law of Equal Areas), can be used to show that the velocity of a planet changes as it moves along its orbit! Kepler’s second law is: The line joining the Sun and a planet sweeps through equal areas in an equal amount of time. The image below links to an animation that demonstrates that when a planet is near aphelion (the point furthest from the Sun, labeled with a B on the screen grab below) the line drawn between the Sun and the planet traces out a long, skinny sector between points A and B.

When the planet is close to perihelion (the point closest to the Sun, labeled with a C on the screen grab below), the line drawn between the Sun and the planet traces out a shorter, fatter sector between points C and D. These slices that alternate gray and blue were drawn in such a way that the area inside each sector is the same. That is, the sector between C and D on the right contains the same amount of area as the sector between A and B on the left. Click on this image to launch the animation in Windows Media Player. Kepler's 2nd Law Credit: Dr. Michael Gallis, Penn State Schuylkill Since the areas of these two sectors are identical, then Kepler's second law says that the time it takes the planet to travel between A and B and also between C and D must be the same. If you look at the distance along the ellipse between A and B, it is shorter than the distance between C and D.

Since velocity is distance divided by time, and since the distance between A and B is shorter than the distance between C and D, when you divide those distances by the same amount of time you find that: A planet is moving faster near perihelion and slower near aphelion. The orbits of most planets are almost circular, with eccentricities near 0. In this case, the changes in their speed are not too large over the course of their orbit. For those of you that teach physics, you might note that really, Kepler's second law is just another way of stating that angular momentum is conserved. That is, when the planet is near perihelion, the distance between the Sun and the planet is smaller, so it must increase its tangential velocity to conserve angular momentum, and similarly, when it is near aphelion when their separation is larger, its tangential velocity must decrease so that the total orbital angular momentum is the same as it was at perihelion. Third Law Kepler had all of Tycho’s data on the planets, so he was able to determine how long each planet took to complete one orbit around the Sun. This is usually referred to as the period of an orbit.

Kepler noted that the closer a planet was to the Sun, the faster it orbited the Sun. He was the first scientist to study the planets from the perspective that the Sun influenced their orbits. That is, unlike Ptolemy and Copernicus, who both assumed that the planets “natural motion” was to move at constant speeds along circular paths, Kepler believed that the Sun exerted some kind of force on the planets to push them along their orbits, and because of this, the closer they are to the Sun, the faster they should move. Kepler studied the periods of the planets and their distance from the Sun, and proved the following mathematical relationship, which is Kepler’s Third Law: The square of the period of a planet’s orbit (P) is directly proportional to the cube of the semimajor axis (a) of its elliptical path. P2 ∝ a3 What this means mathematically is that if the square of the period of an object doubles, then the cube of its semimajor axis must also double. The proportionality sign in the above equation means that: P2 = ka3 where k is a constant number.

If we divide both sides of the equation by a3, we see that: P2 / a3 = k This means that for every planet in our solar system, the ratio of their period squared to their semimajor axis cubed is the same constant value, so this means that: (P2 / a3) Earth = (P2 / a3) Mars = (P2 / a3) Jupiter We know that the period of the Earth is 1 year. At the time of Kepler, they did not know the distances to the planets, but we can just assign the semimajor axis of the Earth to a unit we call the Astronomical Unit (AU). That is, without knowing how big an AU is, we just set aEarth = 1 AU. If you plug 1 year and 1 AU into the equation above, you see that: (P2 / a3) Earth = (1 year)2 / (1 AU)3 = k = 1 So for every planet, P2 / a3 = 1 if P is expressed in years and a is expressed in AU. So if you want to calculate how far Saturn is from the Sun in AU, all you need to know is its period.

For Saturn, this is approximately 29 years. So: (P2 / a3) Saturn = (29 years)2 / (a AU)3 = 1 (a AU)3 = 841 (a AU) = 3√ 841 = 9.4 AU So Saturn is 9.4 times further from the Sun than the Earth is from the Sun!

Question: What do we call the point in the orbit of a planet at which the planet is nearest to the Sun?
Answer: Perihelion